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An example of a 36-hour Tidewatch forecast showing the observed water level (red), the predicted astronomic tide (blue), the difference between the two (the 'residual'; green), and the forecast water level(dotted red line).

All Stations Forecast

The Tidewatch forecast system includes a chart showing extratidal high water at each of the 10 Tidewatch stations.

Tidewatch forecasts go public

Predictions will help Bay-area residents
better prepare for coastal flooding

Researchers at the Virginia Institute of Marine Science have
added forecast capabilities to their network of Tidewatch water-level stations,
giving residents of the lower Chesapeake Bay region a new on-line tool for gauging
the magnitude of coastal flooding in a given location and minimizing its potential
impacts.

The public launch of the Tidewatch forecast system (vims.edu/tidewatch)
comes just in time for the height of the Atlantic hurricane season, which typically
peaks on September 10. A string of hurricanes and strong nor’easters during
recent years, coupled with rising sea level, have brought several episodes of significant
flooding to Tidewater Virginia and other areas of the Bay.

Emeritus professor John
Boon, lead developer of the Tidewatch system, says the forecasts will provide
concerned citizens with “timely guidance on what the time and height of the
next three high waters are expected to be.” He adds “They can use that
information to prepare for coastal flooding, whether that involves gathering
sand bags, moving possessions to higher ground, adjusting mooring lines for
their boat, or choosing an evacuation route.”

VIMS Dean and Director John Wells calls the Tidewatch
forecast system “an outstanding example of translating the results of research
into products that benefit the citizens of the Commonwealth.”

The Tidewatch system now generates 36-hour public forecasts
for 9 water-level stations within Chesapeake Bay and a single station on
Virginia’s seaside Eastern Shore. The forecasts, updated every half hour, were
previously only available on an experimental basis.

Testing and Refinement

Boon and his colleagues at VIMS—associate professor John
Brubaker and assistant research scientist David Forrest—decided to make the
forecasts available to the public after testing and refinements increased their
confidence in the forecasts’ accuracy.

“We worked with Virginia Sea Grant, the National Weather Service, emergency managers, and selected
waterfront property owners to test and
refine the system over the last three years,” says Boon. “Now we think
it’s good to go.”

Their testing included
performance checks in real-time during Hurricane Irene, tropical storm Ernesto,
and the November 2009 nor’easter, as well as “hindcasts” made by feeding data
from 2003—the year of Hurricane Isabel—into their prediction model. The average
expected error for the Tidewatch forecasts is now about 3.5 inches at 36 hours forward
in time, and about 2.5 inches at 12 hours. However, during a tropical storm or
hurricane the error can be several times as large due to rapidly changing wind
speed and direction. The potential for large errors is offset by half-hourly updates
made with the latest observed water levels.

m30 and the Residual

Boon, Brubaker, and Forrest developed the Tidewatch
forecasts in close partnership with National Weather Service researchers at
NOAA headquarters in Silver Spring, Maryland, and in the regional NWS office in
Wakefield, Virginia. Indeed, output from NOAA’s SLOSH model—the official
instrument for forecasting “Sea, Lake, and Overland Surges from Hurricanes”—is
one of the three components of the Tidewatch forecasts. The second is the predicted
astronomical tide—the high and low water levels forecast in the familiar daily
tidal charts.

What sets the Tidewatch
forecasts apart, says Boon, is its third component: inclusion of a 30-day
running average of the observed water level denoted as m30. When m30 is added
to the predicted tide and their sum subtracted from the observed water level, a
difference that meteorologists and oceanographers call the “residual” results,
viewed in Tidewatch through a moving 30-day window. Changes in the size and
sign of the residual produce what Boon terms the “sub-tidal oscillation,” which
Tidewatch shows in near-real time.

“The sub-tidal oscillation is caused mainly by the movement of weather fronts
through our area,” says Boon. “We here in the mid-latitudes typically get a
frontal system moving through every 4 to 5 days or so, depending on the season, with resulting changes
in wind speed, wind direction, and atmospheric pressure.” These meteorological changes affect
sea level, producing rhythmic increases and decreases in water levels that are
largely independent of an approaching hurricane or nor’easter.

“You can think of these ups
and downs as the ‘weather tide’,” says Boon, “with the changing winds that follow
the passage of each front tending to move water around inside and outside the
Bay. We certainly see that in the residuals.” When a major storm visits the
area, he says, “the rise in water level due to the ‘storm surge’ can cause the residual
to grow quite large.”

In addition to depicting sub-tidal
oscillations and storm surge, m30 also accounts for water-level change occurring
at much longer periods spanning months, years, and decades. These changes
include so-called “sea-level anomalies” that happen every few years, as well as
ongoing sea-level rise relative to the land. “If we didn’t use m30”, says Boon,
“long-term variations in sea level would be folded into the residual and become
indistinguishable from storm surge and the sub-tidal oscillation.”

"Extratidal" Water

Tidewatch is also unique in referencing the height of storm
tides to the “highest astronomic tide.” This value, abbreviated in Tidewatch as
“HAT83,” is the highest predicted tidal level in the current version of the tide
tables for a given location. The “83” refers to 1983, the initial year of the 19-year
interval (1983-2001) that NOAA currently uses as a baseline for its official
tide tables. (This baseline, the “National Tidal Datum Epoch,” is periodically updated
as required to account for the
ongoing change in sea level observed at U.S. tide stations.)

Comparing extreme water
levels to HAT, says Boon, best indicates how storm tides will affect coastal
residents, as it provides a natural benchmark that people have used to site
waterfront structures such as docks, boathouses, and dwellings.

"Points just above HAT fall within a zone that appears
dry most of the time,” says Boon. “Coastal residents therefore tend to use that
land, adding infrastructure that enhances its monetary value and encourages its
use. A Tidewatch forecast that calls for water levels to exceed the HAT
line—what we call ‘extratidal’ water—is thus of special concern, as those
conditions are likely to affect vulnerable property.”

Boon says another benefit of referencing storm tides to HAT is
that doing so removes the effect of tidal range—an independent factor that
varies from place to place. “Only extratidal high water is a true index of
flooding severity at two locations with different tidal ranges,” he says.

Boon will explore the public launch of the Tidewatch
forecast system during VIMS’ After Hours Lecture on Thursday, August 23rd.
The lecture begins at 7:00 pm and takes place in Watermen’s Hall on the VIMS
campus in Gloucester Point. Visit https://events.wm.edu/event/view/vims/9580 to register online.